This invention relates generally to radio receivers and, more particularly, to radio receivers used in cellular telephones and to cellular telephone systems.
In cellular telephone communication systems, mobile telephones roam from the geographical limits, or cell, of one base transceiver at which the carrier frequency strength is dominant to another, usually adjacent, cell where the carrier frequency strength of the other cell is dominant. A mobile service center having a land based connection with a land based public telephonic system provides a land based connection with a plurality of base station receivers spatially distributed across a geographical service area associated with the mobile service area. Multiple mobile service centers have service areas in contiguous relationship. Because radio carrier frequency signal strength diminishes quickly with distance from the transmitter, it becomes necessary to provide a “hand-over” procedure in the communication protocol. Pursuant to the hand-over protocol it is determined that a particular mobile telephone has moved from a point within the service area of the cell of a base station to a location at which the carrier frequency signal strength has become weaker than the carrier of an adjacent base station and service of the mobile telephone is transferred from the one base station to the adjacent base station. This transfer, or hand-over, is performed under control of the mobile service center in response to input hand-over signals from the one base station currently servicing the mobile telephone.
In the analog cellular mobile telephone system used in the United States and known as AMPS (American Mobile Phone System), the mobile phones transmit and receive continuously during a call, and no scanning of other base stations for signal strength is performed during a call. This is one type of a continuous communication system. Instead, each base station incorporates a scanning receiver that searches for mobile telephones approaching the associated territorial limits of its service area from an adjacent base station. Thus, base stations in AMPS were burdened with the cost of hardware over and above that required for the communications capability being provided by them.
In digital, noncontinuous, cellular telephone communication systems employing time division multiple access (TDMA), such as the D-AMPS communication system used in the United States and the GSM system used in Europe, time-slotted transmission is employed to allow mobile assisted handover scanning during time periods when there is neither transmission nor reception of informational content carrying communication signals representing the voice messages of the parties to the communication. In these TDMA cellular, noncontinuous, communication systems, during each one of successive periodic communication cycles, or frames, the mobile cellular telephone receives digital data representing coded speech during a first timeslot, transmits digitally coded speech to the base station during a second timeslot, and momentarily retunes the receiver to another frequency to evaluate the signal from another base station during a third time slot. The cycle, or TDMA frame, is repeated at a rate of fifty to three hundred times per second, depending on the system. Only one of the plurality of different base station carrier frequencies is scanned during each TDMA frame which, accordingly, results in fifty to three hundred measurements per second. Recurring measurements of the same carrier frequency are averaged, and the average signal strength of each scanned carrier frequency channel is reported approximately once per second. The mobile telephone sends the scanning report to the currently servicing base station by multiplexing the signal reports with the digitally encoded speech transmitted to the base station during the second, or transmission, period.
In continuous digital communication systems employing frequency division multiple access (FDMA) and code division multiple access (CDMA) protocols, such as the protocol of the IS95 system used in the United States, a return has been made to continuous transmission and reception of the formerly employed analog systems in which there are no periods during which informational content carrying communication signals, or communication signals, are not being transmitted or received. Consequently, as in the case of the prior analog continuous communication systems, in the digital, continuous communication systems, the mobile receiver of the prior art is not able to participate in monitoring for strength of the base station carrier signals of adjacent stations in order to advise the current base station if there is a stronger and probably closer base station to which a handover should be made. The current invention solves this problem by allowing the mobile to make handover measurements without interrupting traffic.